1. Field of the Invention
The invention relates to the field of radioelectric transmission of digital data and relates more particularly to a reception chain comprising a direct microwave demodulation device.
2. Description of the Prior Art
The usual microwave reception chains comprise, in series after the reception antenna, a microwave band filter followed by a low-noise amplifier, then by a mixer performing a transposition into intermediate frequency by means of a local reception oscillator. The chain furthermore comprises an intermediate frequency preamplifier, cells for correcting the group propagation time, and an intermediate frequency amplifier. The circuit for demodulation of the intermediate frequency signal then permits restoring the signal in a base band, the coded signal being restored after processing of the signal in the base band. In such reception chains, the double transposition of microwaves into intermediate frequency and then from the intermediate frequency into the base band, leads to utilising a whole array of intermediate frequency circuits, with a local oscillator in particular.
A description has been given in the FUJITSU periodical, vol. 11, No. 4 of Dec. 1975 KAWASAKI, pages 63 to 67, in an article titled: "Synchronous phase demodulators for high speed quadrature PSK transmission systems" of a microwave reception chain of a more simple structure which makes it possible to perform a direct demodulation on a received microwave signal, with a smaller number of elements, the intermediate frequency circuits being omitted.
In this arrangement, no means are provided for maintaining a constant level at the output of the reception chain. However it is essential to incorporate such a level control.
One obvious solution to this problem consists in providing an automatic gain control upstream of the demodulator, for the microwave signal received. However, a solution of this kind leads to making use of variable microwave "PIN" diodes, which are interposed between microwave amplifiers. Furthermore, the dynamic of the input signal being of the order of 55 dB, it is necessary that the dynamic of the control circuit should be at least 55 dB, which in a solution of this kind requires a costly circuit to be located upstream of the demodulator.